Coulomb's equation
states that in order to slide one hand against the other the
shear stress — akin to the force applied to slide your hands — divided by the surrounding pressure — squeezing the sand together — must equal something called the friction coefficient.
«This unstable
state concludes with an actual labquake, in which the
shearing block rapidly displaces, the friction and
shear stress decrease precipitously, and the gouge layers simultaneously compact,» he said.